Hyperspectral imaging with a spatial heterodyne spectrometer

ABSTRACT

A hyperspectral imaging apparatus based on a monolithic or free space optical spatial heterodyne spectrometer (SHS) design, array detector, electromagnetic radiation source, and optical collection element is described. The apparatus enables the simultaneous acquisition of spatially isolated Fizeau fringe patterns, each having an encoded light product that is decoded to produce a spectral fingerprint of the interrogated object. Features specific to the SHS, such as a large entrance aperture, large acceptance angle, and no moving parts, enable a variety of optical collection schemes including lens arrays, solid-core and hollow core waveguides, and others. In one example, a microlens array (MLA) is configured with the hyperspectral imaging apparatus to simultaneously image many hundred spatially isolated Fizeau fringe patterns while interrogating an object using an electromagnetic radiation source. Each Fizeau fringe pattern recorded by the array detector is decoded to produce a full Raman or laser-induced breakdown spectroscopy (LIBS) spectrum. Compared to prior art, the hyperspectral imaging apparatus overcomes the primary limitations of needing to trade time resolution for both spectral and spatial data density because the imaging apparatus simultaneously acquires both spectral and special information. Based on the selection and configuration of diffraction gratings, the grating aperture size, Littrow wavelength (i.e., heterodyne wavelength), and optical collection configuration, the apparatus can be tailored to produced low or high spectral resolution with a spectral bandpass that covers a portion or the entire Raman spectral range (up to 4200 cm −1 ) and for LIBS as well.

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationSer. No. 62/933,732, filed on Nov. 11, 2019, which is incorporatedherein in its entirety by reference thereto.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH

This invention was made with Government support under Grant No.OCE1829333, awarded by the National Science Foundation, and Grant No.80NSSC19K1024, awarded by the National Aeronautics and SpaceAdministration. The Government has certain rights in the invention.

BACKGROUND

There is a continuing need for optical systems that can interrogate amixed sample and identify chemical constituents, particularly when thesample is heterogeneous, and the interrogation can be done withoutsample preparation. The interaction of light with a sample byscattering, absorption and emission (i.e., light products) providesspectral ‘fingerprints’ that can be used to identify and quantify thechemical composition in a sample. Optical interrogation systems enablenew off-line, real-time, and in-situ, on-line, or in-line measurementcapabilities that benefit many branches of science including biology,medicine, material, forensics, and chemistry. One specific area that hasbenefited greatly from advances in measurement science is industrialprocess analysis, which supports process applications in thepharmaceutical industry, the manufacturing of materials, and as part ofthe feedback and control in certain chemical processes. Of particularinterest is mapping the identity, concentrations, and temporal and/orspatial changes of chemical constituents using spectroscopic imaging.

Hyperspectral imaging is the practice of recording multiple spectralband intensities for several select spatial points on an object (i.e.,sample). Hyperspectral imaging has been described using Ramanspectroscopy, laser-induced breakdown spectroscopy (LIBS), as well asluminescence. To date, hyperspectral Raman techniques have relied onscanning instruments, where spectra are acquired using a spectrometerfor one spatial point at a time, as the excitation laser is scanned frompoint to point on the sample. The spectra for every point are thenmapped to generate a map of spectral intensity as a function ofwavelength and position, the hypercube of data. For point-by-pointscanning, the laser can be focused to a small spot, and the sampleraster scanned through the laser beam, collecting a spectrum at eachpoint. Alternatively, the laser can be focused to a thin line, usingcylindrical optics, and the Raman scattering imaged onto the slit of adispersive spectrometer. Another approach is to illuminate the wholesample using an expanded laser spot and a series of two-dimensional (2D)images are acquired using a tunable filter. Various types of filtershave been used in this method, including dielectric filters,acousto-optic, and liquid-crystal tunable filters. In another approach,fiber optic arrays have been used, where spatial and spectralinformation were simultaneously acquired. However, the number of spatialpoints demonstrated using a fiber array is low and the fiber fill factoris also low providing a relatively weak signal. The prior art hasseveral limitations, particularly the need to trade time resolution forboth spectral and spatial data density.

There is a need for an apparatus and method enabling rapid andsimultaneous image acquisition with complete spectral information. Thepresent inventors have demonstrated that Spatial Heterodyne RamanSpectrometers (SHRS) and Spatial Heterodyne LIBS Spectrometers (SHLS)are especially suited for addressing the limitations described above andfor enabling encoding the light product received from an object toobtain low-resolution or high-resolution spectra simultaneously at manyspatially isolated locations on the object. This is possible because thespatial heterodyne spectrometer (SHS) has the unique property ofallowing many spatially isolated beams of light to be simultaneouslymeasured, by illuminating each beam of light onto a separate region ofthe SHS diffraction grating or gratings, or other alternate dispersiveelement, and onto separate regions of the charge-coupled device (CCD),intensified charge-coupled device (ICCD), complementary metal-oxidesemiconductor (CMOS) detector or other type of array detector. As aproof of demonstration, the present inventors have used a microlensarray (MLA) with a SHRS for the simultaneous acquisition ofhyperspectral images resulting in a complete Raman spectrum at numerousspatially isolated object locations using a single acquisition or singlelaser pulse. This allows complete image acquisition with correspondingcomplete spectral information simultaneously and in seconds to minutesrather than hours that can be required using current, prior arthyperspectral Raman techniques. Furthermore, use of a single acquisitionor single laser shot (e.g., laser pulse) mitigates degradation of theobject (i.e., sample) as might occur from repeated or prolonged exposureto intense laser light.

It can be advantageous to isolate the array detector from the othercomponents of the SHRS and SHLS. Fiber optic waveguides can be used forthis purpose. Fiber optic waveguides can also be used to transfer theimage of the MLA to the input of the SHRS and SHLS.

In addition to utilizing MLAs or waveguides, further improvements can bemade to the hyperspectral design by incorporating features to increasethe robustness and performance. Monolithic SHRS and SHLS designs, asopposed to free space (e.g., where components are individually mounted)optical SHS configurations, are fabricated from bonded (e.g., opticallycontacted, epoxied, cemented) components of quartz, fused silica, BK7,other types of glasses or combinations thereof. This maintains thepre-alignment of components and greatly expands their potential use forapplications such as, for example, industrial process analysis. Themonolithic design, which can be made small relative to a conventionalspectrometer of similar resolving power, can enable simultaneousmeasurements of many processes or redundant measurements of the sameprocess, similar to imaging described above. This can be achieved whilestill maintaining a large spectral range by the use of array detectorswith very small pixel dimensions, such as CMOS detectors. The spectralrange of the SHRS has also been shown to be approximately doubled byusing a 2D SHS, where one grating is tilted vertically to remove theredundancy of Raman bands above and below Littrow. Alternatively, as weshow in this disclosure, one grating can instead be rotated about thegrating normal, as opposed to tilted, to remove the said degeneracy toachieve the similar approximate doubling of the spectral range. This hasthe added benefit of improving manufacturability and assembly/alignmentof the monolithic SHS device.

SUMMARY OF THE INVENTION

Objects and advantages of the present invention will be set forth inpart in the following description, or may be obvious from thedescription, or may be learned through practice of the invention. Theinvention will be described in greater detail below by reference toembodiments thereof illustrated in the figures.

The present invention provides embodiments of an optical device andapparatus that enable hyperspectral measurements, which is describedherein as producing and simultaneously acquiring spatially isolatedFizeau fringe patterns each having an encoded light product that can bedecoded to produce a spectral fingerprint of the interrogated object(i.e., sample).

In one particular embodiment, the present invention includes an opticalapparatus for producing and simultaneously acquiring at least twospatially isolated Fizeau fringe patterns each having an encoded lightproduct formed as a result of receiving a light product from at leastone object. Further, the optical apparatus includes at least one spatialheterodyne spectrometer constructed to receive at least two light inputbeams and produce, from each said light input beam, two correspondinglight output beams of said spatially isolated Fizeau fringe patterns;wherein the at least one spatial heterodyne spectrometer comprises abeam splitter for directing the light product and subsequentlyrecombining, and one or more diffraction gratings, wherein thediffraction gratings are configured to adjust a wavelength of the lightproduct; an optical element for receiving the light product from the atleast one object and produce the at least two light input beams to theat least one spatial heterodyne spectrometer; a means for directing atleast one excitation source to interact with the at least one object toproduce the light product; and at least one detector array and at leastone optical element for imaging the at least two spatially isolatedFizeau fringe patterns.

In another embodiment, the present invention is directed to a device forimaging a sample. The device includes comprising an excitation source; aspatial heterodyne spectrometer; and a microlens array; wherein themicrolens array and a surface of the sample to be imaged are arranged inparallel, and wherein the microlens array collects light from differentregions of the surface of the sample.

In still another embodiment, the present invention also contemplates amethod for forming a hyperspectral image via spatial heterodyne Ramanspectroscopy. The method includes illuminating a sample with wavelengthsfrom an excitation source; utilizing a microlens array, wherein themicrolens array and a surface of the sample to be imaged are arranged inparallel, and wherein the microlens array collects light from differentregions of the surface of the sample; and utilizing a spatial heterodynespectrometer configured to receive a signal from the microlens array.

In yet another embodiment, the present invention contemplates a devicethat includes an excitation source; a spatial heterodyne spectrometercomprised of a beam splitter and a pair of diffraction gratings; and oneor more additional diffraction gratings.

In an additional embodiment, the present invention provides a method forspectroscopy that includes illuminating a sample with wavelengths froman excitation source; utilizing a spatial heterodyne spectrometercomprised of a beam splitter and a pair of original diffractiongratings; and utilizing one or more additional diffraction gratings toincrease the spectral range or measure two spectral ranges.

Embodiments of the present invention include spatial heterodynespectrometers (SHS) constructed to receive at least two light inputbeams and produce two corresponding light output beams for each lightinput beam that, upon recombination, produce spatially isolated Fizeaufringe patterns. The SHS may also encompass an optical filter(s) andcoating(s) and/or a spatial filter(s). An optical element consisting of,but not limited to, a lens, lens array, MLA, or one of several types ofwaveguides that acts as a means for receiving the light product (e.g.,Raman and/or LIBS wavelengths) from the object and for delivering to theSHS entrance aperture. The optical element may also encompass an opticalfilter(s) and a coating(s) and/or a spatial filter(s). An excitationsource (e.g., a source of light, a source of electromagnetic radiationincluding, but not limited to, a laser source, light emitting diode)interacts with the object to produce the light product (i.e., Ramanand/or LIBS wavelengths). The use of a single or multiple detector arrayis for recording the spatially isolated Fizeau fringe patterns. Theembodiments using a first-rotated or first-tilted grating with respectto a second grating enhance the spectral range by inducing a phase shiftalong the y-axis, enabling differentiation of spectra on either side ofthe Littrow (i.e., heterodyne) wavelength via decoding with a 2D Fouriertransform method, which may result in an approximate doubling of thespectral range.

A variety of spectroscopic techniques can be performed with thishyperspectral apparatus separate or in parallel including, but notlimited to, Raman and laser-induced breakdown spectroscopy (LIBS).

In one particular embodiment of the present invention, an image transferoptical element including, but not limited to, optical fibers, fiberoptic image conduit, fiber optic taper, fiber optic faceplate, or othercoherent arrangement is used to relay the spatially-isolated Fizeaufringe patterns to the detector array, which may be located somedistance away from the SHS.

In one particular embodiment of the present invention, a device forsimultaneously imaging spatially isolated Fizeau fringe patterns from asample is provided. The device includes an excitation source (e.g., alight source, source of electromagnetic radiation, etc.), a spatialheterodyne spectrometer, and a microlens array (MLA), where the MLA isarranged in line of sight of the sample to be measured, and where theMLA collects light from different regions of the sample.

In one embodiment, the excitation source can be a light emitting diode,laser source, coherent source, incoherent source, or combinationsthereof.

In another embodiment, the spatial heterodyne spectrometer is configuredto receive emission wavelengths from the sample. For example, the devicemay include one or more band pass filters and blocking filters in suchan embodiment, which are configured to remove the light outside theemission wavelengths.

In yet another embodiment, the spatial heterodyne spectrometer isconfigured to receive Raman wavelengths from the sample. For example,the device may include one or more band pass filters and blockingfilters in such an embodiment, which are configured to remove the lightoutside the Raman wavelengths.

In one more embodiment, the device is further comprised of acharge-coupled device configured to collect Raman wavelengths oremission wavelengths or combinations thereof.

In another embodiment, the spatial heterodyne spectrometer is furthercomprised of two diffraction gratings or two dispersive prisms, thediffraction gratings or dispersive prisms configured to adjust Ramanwavelengths. For example, in such an embodiment, the Littrow angle ofthe diffraction gratings or the dispersive prisms is adjustable. Also,in a different such embodiment, the spatial heterodyne spectrometer isfurther comprised of one or more prisms to adjust an acceptance angle ofthe light from the sample.

In another embodiment, the spatial heterodyne spectrometer is furthercomprised of a diffraction grating or dispersive prism and at least onereflective optic (e.g., mirror), the diffraction grating or dispersiveprism and reflective optic configured to adjust Raman wavelengths. Forexample, in such an embodiment, the Littrow angle of the diffractiongrating or the dispersive prism is adjustable. Also, in a different suchembodiment, the spatial heterodyne spectrometer is further comprised ofone or more prisms to adjust an acceptance angle of the light from thesample.

In yet another embodiment, the device contains transfer optics, definedas one or more collection lenses or apertures, for directing the lightproduct from the MLA to within an acceptance angle of the spatialheterodyne spectrometer. As an example, in such an embodiment, a relaylens may be positioned two focal lengths from the MLA and two focallengths from an aperture of the spatial heterodyne spectrometer. In adifferent such embodiment, the center of the MLA must be aligned withthe center of the relay lens.

In an additional embodiment, the MLA magnifies an image of the sample.

In one more embodiment, a method for forming a hyperspectral image usingspatial heterodyne Raman spectroscopy is provided. The method includesilluminating a sample with wavelengths from an excitation source (e.g.,a light source, source of electromagnetic radiation, etc.); utilizing aMLA, where the MLA is in line of sight of the sample to be measured, andwhere the MLA collects light from different regions of the sample; andutilizing a spatial heterodyne spectrometer.

In one embodiment, the excitation source can be a light emitting diode,laser source, coherent source, incoherent source, or combinationsthereof.

In yet another embodiment, the spatial heterodyne spectrometer isconfigured to receive Raman wavelengths (i.e., light product) from thesample. For example, the method may include the use of one or more bandpass filters and blocking filters in such an embodiment, which areconfigured to remove the light outside the Raman wavelengths.

In still another embodiment, the MLA adjusts the light product from thesample into unique spatially resolved regions and directs the lightproduct to the spatial heterodyne aperture. For example, in one suchembodiment, Fourier transform methods are performed upon the uniquespatially isolated Fizeau fringe patterns recorded by the arraydetector.

In another embodiment, Raman spectra, for points (i.e., spatiallyisolated Fizeau fringe pattern) in the hyperspectral image, are decodedusing a Fourier transform method.

In yet another embodiment, the method includes the use of transferoptics, which included, but is not limited to, one or more collectionlenses or apertures for directing the light product from the object fromthe MLA to within an acceptance angle of the spatial heterodynespectrometer. For example, in such an embodiment, a relay lens may bepositioned two focal lengths from the MLA and two focal lengths from anaperture of the spatial heterodyne spectrometer. In a different suchembodiment, the center of the MLA can be aligned with the center of therelay lens.

In yet another embodiment, the excitation source is a pulsed laser.

In one particular embodiment of the present invention, a device forhyperspectral imaging across an expanded portion of the SHS gratingaperture width and height dimension is provided. The device includes anexcitation source and a spatial heterodyne spectrometer comprised of abeam splitter and a pair of diffraction gratings with additionalgratings positioned adjacently such that the periodic structured surfaceof each adjacently positioned grating is stepped (i.e., offset apredetermined distance) to adjust for pathlength differences across aportion of or across the entire grating aperture along the dimensionparallel or perpendicular to the grating dispersion, where it is to beunderstood that the grating aperture refers to the grating area that isuseable in the SHS, which in some examples represents apertures ofapproximately 17×17 mm², although other grating areas are contemplatedby the present invention as would be understood by one of ordinary skillin the art. Adjusting for pathlength differences along the dimensionparallel to the grating dispersion is the preferred implementation for anon-tilted (i.e., rotated) grating SHS design. Adjusting for pathlengthdifferences along the dimension parallel and perpendicular to thegrating dispersion is the preferred implementation for a tilted (i.e.,non-rotated) grating SHS design. This is more challenging to implementfor the tilted grating, which is one reason for utilizing a rotatedgrating approach as disclosed herein.

In another particular embodiment of the present invention, theaforementioned stepped approach is accomplished in a different way wherethe SHS comprises two single gratings having one or more steppedperiodic structured surfaces. Further, it is also to be understood thatthe aforementioned adjustment for pathlength differences across aportion of or across the entire grating aperture can be accomplished ina different way where a refractive optic is configured between the beamsplitter and diffraction gratings.

In one embodiment, the excitation source can be a light emitting diode,laser source, coherent source, incoherent source, or combinationsthereof.

In another embodiment, the spatial heterodyne spectrometer is amonolithic spatial heterodyne spectrometer.

In yet another embodiment, one or more additional diffraction gratingsare adjacently positioned.

In one other embodiment, a different Littrow wavelength for each of theadditional diffraction gratings is selected by adjusting a grating angleof each of the additional diffraction gratings individually relative toeach other and relative to the grating angle of the pair of diffractiongratings in the spatial heterodyne spectrometer.

In another embodiment, each of the additional diffraction gratings has aunique groove density relative to each other and relative to the groovedensity of the pair of diffraction gratings in the spatial heterodynespectrometer.

In another embodiment, the encoded light product can be decoded using aFourier transform method or other decoding methods.

In one particular embodiment, a method for spectroscopy is provided. Themethod includes illuminating a sample with wavelengths from anexcitation source; utilizing a spatial heterodyne spectrometer comprisedof a beam splitter and a pair of diffraction gratings; and utilizing oneor more additional diffraction gratings.

In another embodiment, the additional diffraction gratings areadjacently positioned.

Also, in a further embodiment, a different Littrow wavelength for eachof the additional diffraction gratings is selected by adjusting thegrating angle of each diffraction grating individually relative to eachother and relative to the grating angle of the diffraction gratings ofthe spatial heterodyne spectrometer.

In a different embodiment, each of the additional diffraction gratingshas a unique groove density relative to each other and relative to thegroove density of the pair of diffraction gratings of the spatialheterodyne spectrometer.

In yet another embodiment, the method further comprises performingFourier transforms of the image.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying figures, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention to one skilledin the art, including the best mode thereof, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures, in which:

FIG. 1A illustrates one embodiment of a spatial heterodyne Ramanspectrometer with its constituent components that is contemplated foruse in the present invention.

FIG. 1B illustrates the Raman spectra results from a traditional SHRSfor (I) a diamond in potassium perchlorate pellet; (II) a sodium sulfateand potassium perchlorate bilayer pellet; (III) a sodium nitrate andpotassium perchlorate bilayer pellet; and (IV) an acetaminophen andammonium nitrate bilayer pellet.

FIG. 1C illustrates the Littrow configuration of a diffraction gratingin a SHRS.

FIG. 1D illustrates a path of light (i.e., photons) that strikes adiffraction grating in a SHRS at a wavelength other than the Littrowwavelength.

FIG. 2A shows one microlens array (MLA) contemplated for use in thesystem of the present invention. The MLA is a 40 lenslet by 40 lensletMLA with 1600 lenslets total. The lenslets have a diameter of 100 μmeach and a focal length, f_(m), of 1.4 mm or f/16, for a 1×magnification setup. The inset image shows a 6 row by 4 column magnifiedsection of the MLA.

FIG. 2B illustrates a detailed diagram of the present inventionconfigured with an MLA. The MLA of FIG. 2A or any other suitable MLA orwaveguide of various types is located before the SHS entrance apertureand in line of sight of the sample. The light product originates fromsample, which is being interrogated by the source of electromagneticradiation and collected at the focal point of the MLA, allowing lightbeams to exit the back surface of the MLA. The light beams from eachlenslet remains separated in space allowing spatial-spectral informationto be retained. The light path is focused by another lens beforeentering the SHRS. Filters of various types (not illustrated) are alsopresent in the optical path.

FIG. 2C is an overview of the entire device including the spatialheterodyne spectrometer, the MLA, and the associated relay optics.

FIG. 2D is a side view of the device showing the laser and MLA setup.

FIG. 3 illustrates the data processing method involved in the presentinvention.

FIG. 4 is a fringe image (i.e., Fizeau fringe pattern) of a diamondsample recorded using the SHRS array detector. The square outline 3 rowsdown and five columns across indicates a lenslet of interestcorresponding to a region of interest, which is shown to be spatiallyisolated.

FIG. 5 is a graph of the sum of the columns of the fringe image from thelenslet of interest of FIG. 4 . The smoother line is a high orderpolynomial fit of the data.

FIG. 6 is a graph of the interferogram cross section result aftersubtracting the high order polynomial from the data of FIG. 5 .

FIG. 7 is a plot of the spectrum extracted after taking a 1D Fouriertransform of the interferogram of FIG. 6 . This shows the Raman spectrumof the diamond sample.

FIG. 8 shows (top) the raw interferogram recorded on the SHRS arraydetector of a sample of diamond in a potassium perchlorate pellet withthe labeled regions of interest A, B, and C with (bottom: inset) thecorrected interferogram cross section for each region of interest A, B,and C and the corresponding Raman spectra (bottom) extracted via aFourier transform method from each corrected interferogram. Region ofinterest A shows a single prominent Raman band for diamond; B showsprominent Raman bands for diamond and potassium perchlorate; and C showsa single prominent Raman band for potassium perchlorate.

FIG. 9 shows (top) the raw interferogram recorded on the SHRS arraydetector of a sample of sodium sulfate and potassium perchlorate bilayerpellet with the labeled regions of interest A, B, and C with (bottom:inset) the corrected interferogram cross-section for each region ofinterest A, B, and C and the corresponding Raman spectra (bottom)extracted via a Fourier transform method from each correctedinterferogram. Region of interest A shows a single prominent Raman bandfor potassium perchlorate; B shows prominent Raman bands for potassiumperchlorate and sodium sulfate; and C shows a single prominent Ramanband for sodium sulfate.

FIG. 10 shows (top) the raw interferogram recorded on the SHRS arraydetector of a sample of sodium nitrate and potassium perchlorate bilayerpellet with the labeled regions of interest A, B, and C and thecorresponding Raman spectra (bottom) extracted via a Fourier transformmethod from each corrected interferogram (not shown). Region of interestA shows a single prominent Raman band for sodium nitrate; B showsprominent Raman bands for potassium perchlorate and sodium nitrate; andC shows a single prominent Raman band for potassium perchlorate.

FIG. 11 shows (top) the raw interferogram recorded on the SHRS arraydetector of a sample of acetaminophen and ammonium nitrate bilayerpellet with the labeled regions of interest A, B, C, and D and thecorresponding Raman spectra (bottom) extracted via a Fourier transformmethod from each corrected interferogram (not shown). Region of interestA shows prominent Raman bands for acetaminophen; B and C show prominentRaman bands for acetaminophen and ammonium nitrate at different relativeconcentrations; and D shows a prominent Raman band for ammonium nitrate.

FIG. 12 illustrates the tradeoff between the spectral resolution (i.e.,full width half maximum—FWHM—of the Raman spectral feature for diamond)and the area viewed on the diamond sample, of FIG. 8 , where increasingthe number of horizontal lenslets (e.g., grating grooves illuminated) ofthe region of interest that undergo a Fourier transform, results in asmaller FWHM value.

FIG. 13 is a picture of two side-by-side monolithic spatial heterodynespectrometers (SHSs) designed for use as SHRS and SHLS devices. Each SHSdevice is comprised of the beam splitter, diffraction gratings, gratingholders and spacer components that are fused via optical contact orepoxy. Both monolith SHSs shown here are constructed of BK7 material.

FIG. 14 is a diagram of a spatial heterodyne spectrometer designed withmultiple additional diffraction gratings positioned at the same Littrowangle, OL. A spacer or spacers can be added to compensate for thedifferent amount of space between the diffraction gratings. One or morepairs of diffraction gratings can be added and adjacently positioned tothe original pair.

FIG. 15 is a diagram of a spatial heterodyne spectrometer with wedgeprisms used to increase the acceptance angle of the incoming signal.

FIG. 16 shows a 4×4 section of lenslets having spatially isolatedinterferograms with a cross-hatched pattern, which is indicative of arotation of the Fizeau fringes and a phase shift along the y-axis.

FIG. 17 shows how a refractive corrective optic (RCO) can be used tomitigate SHS pathlength differences across the grating aperture, wherethe grating aperture refers to the grating area that is useable in theSHS.

DETAILED DESCRIPTION

Reference now will be made to the embodiments of the invention, one ormore examples of which are set forth below. Each example is provided byway of an explanation of the invention, not as a limitation of theinvention. In fact, it will be apparent to those skilled in the art thatvarious modifications and variations can be made in the inventionwithout departing from the scope or spirit of the invention. Forinstance, features illustrated or described as one embodiment can beused on another embodiment to yield still a further embodiment. Thus, itis intended that the present invention cover such modifications andvariations as come within the scope of the appended claims and theirequivalents. It is to be understood by one of ordinary skill in the artthat the present discussion is a description of exemplary embodimentsonly and is not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied exemplary constructions.

As used herein, the terms “about,” “approximately,” or “generally,” whenused to modify a value, indicates that the value can be raised orlowered by 5% and remain within the disclosed embodiment.

The present invention greatly improves the usability and practicabilityof prior art spatial heterodyne spectrometers (SHSs) and, in particular,spatial heterodyne Raman spectrometers (SHRSs) and spatial heterodyneLIBS spectrometers (SHLSs) by first, enabling the equivalent approximatedoubling of the spectral range using the new rotated grating approachversus the prior art tilted grating approach, thereby maintaining theequivalent SHS pathlength differences along the vertical (perpendicularto the grating dispersion) dimension to enable hyperspectral imagingpreferably over the entire vertical dimension of the grating aperture ofthe SHS device. Second, the stepped grating SHS design approachdisclosed herein maintains the equivalent SHS pathlength differencesalong the horizontal (parallel to the grating dispersion) dimension toenable hyperspectral imaging preferably over the entire horizontaldimension of the grating aperture of the SHS device. Third, when thestepped grating SHS design approach is combined with the rotated gratingSHS design approach into a single SHS device, hyperspectral imaging isenabled preferably over the entire vertical and horizontal dimensions ofthe grating aperture of the SHS device while also enabling theapproximate doubling of the spectral range.

The prior art for hyperspectral techniques has several limitations, inparticular the need to trade time resolution for both spectral andspatial data density. The present invention overcomes the limitations ofall prior art hyperspectral techniques by enabling simultaneousacquisition of hyperspectral images resulting in a complete Raman (orLIBS) spectrum at numerous spatially isolated object locations using asingle acquisition or single laser pulse. This allows complete imageacquisition with corresponding complete spectral informationsimultaneously. Furthermore, use of a single acquisition or single lasershot (e.g., laser pulse) mitigates degradation of the sample as mightoccur from repeated or prolonged exposure to intense laser light.

Specifically, the present invention enables encoding the light productreceived from an object in a single acquisition or single laser pulse toobtain low- or high-resolution spectra simultaneously at many spatiallyisolated locations on the sample. This is possible because the SHS hasthe unique property of allowing many spatially isolated beams of lightto be simultaneously measured, by illuminating each beam of light onto aseparate region of the SHS diffraction grating or gratings, or otheralternate dispersive element, and onto separate regions of the CCD,ICCD, CMOS detector, or other type of array detector.

In one embodiment, we show that by using a microlens array (MLA) incombination with a SHS, the present invention can capture data for aplethora of points on the sample with a single acquisition or singlelaser shot. Furthermore, additional diffraction gratings can be addedand imaged in order to enable hyperspectral imaging across an expandedportion of the SHS grating aperture by adjusting pathlength differences.Furthermore, one grating can be rotated about the grating normal, asopposed to tilting, to enable an approximate doubling of the spectralrange. These improvements represent a significant advancement over allthe prior art.

In an exemplary embodiment, the present invention contemplates the useof a (SHRS) (100), as depicted in FIG. 1A. The SHRS (100) is adispersive interferometer that uses a pair of stationary reflectivediffraction gratings (102 a, 102 b). The light beam(s) (112 b, 118)enters the input aperture (106) of the SHRS where it is split into twooutput beams (112 c, 112 d) by a preferably 50:50, beamsplitter (101).These two output beams strike the stationary diffraction gratings (102a, 102 b), which are adjusted at a predetermined Littrow angle, θ_(L),such that one particular wavelength is retro-reflected along theincident light path (112 c and 112 d) and recombines at the beamsplitter(101). As shown in FIG. 1C, heterodyning in the interferometer occurs atthe Littrow wavelength, λ_(L), corresponding to the wavelength of lightthat is exactly retro-reflected back along the same path (121), andhence, recombines at the beamsplitter (101, see FIG. 1A) withoutinterference. The Littrow configuration angle (θ_(L)) is determinedrelative to a line normal (122) to the surface of the diffractiongrating (125). When in a Littrow configuration, the Littrowconfiguration angle, θ_(L), will be equal to the angle of the blazings,θ_(B), and will also be normal (121) to the blazed surface (127). Also,in such a configuration, the incident angle (θ_(I)) and the diffractionangle (θ_(D)) are equal to the Littrow configuration angle (θ_(L)). TheLittrow configuration angle and the Littrow wavelength are related byEquation 3 below. For any wavelength other than the Littrow wavelength,as illustrated in FIG. 1D, the incident light (123) strikes the surface(127) of the periodic structured blazings (126) at an angle (θ_(I)) tothe optical axis (122). Diffracted light (124) leaves the surface (127)of the blazings (126) on the diffraction gratings (102) at an angle(θ_(D)) to the optical axis (122), resulting in crossed wavefronts,inducing a spatial phase shift, and generating an interference pattern,which produces a series of wavelength dependent fringes on the arraydetector. The fringe spatial frequency on the detector is given byEquation 1, where f is in fringes/cm and σ is the wavelength expressedin wavenumbers. A Fourier transform of the interferogram decodes andrecovers the spectrum.

f=4(σ−σ_(L))tan θ_(L)  (1)

Ω_(max)=2π/R  (2)

θ_(L)=arcsin(mλ _(L)/2d)

According to Equation 1, emission lines above or below the Littrowwavelength may show identical fringe patterns and can lead to degeneratelines (i.e., line overlap). This degeneracy can be removed by tiltingone of the gratings vertically, which induces a rotation to theinterferogram (i.e., Fizeau fringe pattern), in opposite directionsabove and below Littrow. In this case, a 2D Fourier transform can beused to recover spectra above and below the Littrow wavelengthunambiguously. This technique can be used to approximately double thespectral range of the SHRS.

In an alternative embodiment, the present invention utilizes a gratingrotation, where the grooves of the grating are rotated with respect tothe grooves on the other grating, which are vertical. While there aredifferent ways to achieve this grating rotation—preinstalling thegrating in the glass housing with the desired rotation or preinstallingthe grating with vertical grooves and then rotating the glasshousing—the key concept here is that one set of grooves are rotated (andnot tilted) versus the other set of grooves.

The SHRS, like other Fourier transform interferometers, does not requirea narrow slit to achieve high resolution as is common with dispersivespectrometers because there is only a weak dependence of resolution onentrance aperture width. This allows the SHRS to employ very largeentrance apertures, greatly increasing the throughput of the system,which is advantageous when signal strength of the light product is lowand also for imaging applications.

In the described embodiment of the SHRS (100), the resolving power isequal to the number of grating grooves illuminated, in this caseR=10,800, giving a theoretical resolution of about 0.05 nm (1.7 cm⁻¹) at532 nm. The active area of the charge coupled device (CCD) detector(105) is about 16.2 mm or about 1200 pixels; therefore, the theoreticalspectral range of the SHRS, based on the Nyquist criteria of 2 pixelsper wavelength, is about 30 nm (1073 cm⁻¹). The useful spectral range isonly about ⅔ this value because the instrument response drops quickly atwavelengths far from the Littrow setting. This spectral range can beroughly doubled (to about 60 nm or 2145 cm⁻¹) by tilting or by rotatingone of the gratings slightly (with respect to the other grating) andusing a 2D Fourier transform to recover wavelengths both above and belowthe Littrow wavelength, unambiguously. The maximum, resolution-limitedsolid angle field of view (FOV) of the SHRS is related to the resolvingpower by Equation 2 above. Thus, the solid angle FOV for an exemplaryembodiment without field widening prisms is about 5.8×10⁻⁴ sr, and thefull acceptance angle is about 1.4°. The addition of field wideningprisms, another embodiment, offers several advantages including anincreased acceptance angle.

FIG. 1B shows Raman spectra measured by the traditional SHRS (100) for(I) a diamond in potassium perchlorate pellet, (II) a sodiumsulfate/potassium perchlorate bilayer pellet, (III) a sodiumnitrate/potassium perchlorate bilayer pellet, and (IV) anacetaminophen/ammonium nitrate bilayer pellet. The pellets wereilluminated with 150 mW of continuous 532 nm laser at the interfacewhere the two solids meet. The measured resolution for these Raman bandswas about 7 cm⁻¹ using 300 gr/mm gratings. In this example, the 2Dspatial information was lost and a mean spectrum across the image isobtained.

The present invention is shown in the system (200) in FIG. 2B andutilizes the SHRS (100) shown in FIG. 1A. An area (111 a) of the sampleto be imaged (111) is located at the focal point of an MLA (110), f_(m),which is both illuminating the sample and collecting the signal of thelight product. When a relay lens (108) images the back surface of theMLA (110) onto the SHRS diffraction grating (102 a, 102 b) faces, thelight beams from each lenslet remains separate in space (112 b, 118).Because the SHRS has a large entrance aperture (106), the individuallenslet light beams can enter the SHRS without interfering with eachother (112), which means that spatial-spectral information is retained.When the array of lenslet light beams (118) enters into and is acted onby the SHRS, an interference pattern (i.e., Fizeau fringe pattern) formsfor each individual lenslet. Without the MLA, spatial information islost, and the mean spectrum is obtained.

It is critical that the light enters the SHRS (100) aperture (106)either collimated or within the acceptance angle of the SHRS (100). Thiscan be achieved using a variety of different embodiments. In thedemonstrated embodiment of FIG. 2B, the relay lens (108) is two focallengths from the MLA (110). The MLA lenslets have a focal length, f_(m),which is the distance the lenslets are from the sample. In someembodiments, the center of the MLA (110) must align with the focal pointof the relaying lens (108). The relay lens (108) is also positioned twofocal lengths from entrance aperture (106) of the SHRS (100) and focusesthe light at a point one focal length from the entrance aperture (106).

In one embodiment, shown in FIG. 15 , wedge prisms (119) can be used inthe SHRS (100) in order to increase the acceptance angle. This canincrease the acceptance angle from about 0.1° to about 20°, about 0.5°to about 15°, or about 2.5° to about 10°. Other embodiments of the abovetransfer lens setup (114) would be apparent to those familiar in the artand could include two or more relaying or collimating lenses. Increasingthe acceptance angle allows for the light from each lenslet to remainseparate in space. Without a wide acceptance angle, the light of eachlenslet would overlap, and individual spectrums for different points onthe sample could not be obtained.

As shown in FIG. 2A, the MLA (110) can contain a number of differentlenslets depending on the necessary spatial resolution. An MLA (110) ina 2 lenslet by 2 lenslet configuration yields 4 individual beams. Otherembodiments shown here include up to a 40 lenslet by 40 lenslet arraywith 1600 lenslets total. Still further embodiments can have lensletarrays ranging between 2×2 to 1000×1000, 3×3 to 200×200, and 4×4 to100×100. The magnification of the MLA can vary depending on theapplication. Changing the magnification allows control over performancecharacteristics such as the area viewed on the sample or resolvingpower. Magnification in various embodiments could range from about 1× toabout 100×, such as from about 5× to about 50×, such as from about 8× toabout 12×. The lenslets can also have a variety of diameters rangingfrom about 25 microns to about 175 microns, such as from about 50microns to about 150 microns, and such as from about 75 microns to about125 microns with focal lengths can vary over a very wide range.

In an embodiment where optical fibers are used rather than an MLA, therange of individual fiber diameters could be from several micrometers(i.e., microns) to many hundreds of micrometers to a few millimeters. Ofcourse, the size of the fiber diameter(s) used would limit, in part, thenumber of spatially isolated Fizeau fringe patterns obtained on thearray detector.

FIG. 2C shows a layout of the entire MLA-SHRS setup of system 200. Theexcitation light travels from the laser (113, see FIG. 2D) along theexcitation light path (112 a) through a dichroic mirror (109) where itis redirected to the MLA (110). The light is focused by the MLA (110)onto the area (111 a) of the sample (111) at f_(m) where the signal isreflected back along (112 b) through the MLA (110) and through thedichroic mirror (109). Light then travels through the transfer lenssetup (114) which is composed of a relay lens (108) located two focallengths from the MLA (110). The light travels through the relay lens(108) which is two focal lengths from the aperture (106) of the SHRS(100). The relay lens (108) focuses the light at a point one focallength from the aperture. Along this light path (112), between the relaylens (108) and the aperture (106), can be one or more filters (107).These filters (107) are often band pass filters or an alternative typeof blocking filter and are used to block light from outside the Ramanwavelengths. As shown, in FIG. 2C, the filters (107) can be placed inthe collimated portion of the beam (i.e., normal incidence) between thebeam splitter (101) and the aperture (106), or between the aperture(106) and the relay lens (108) at higher f-numbers. The light thentravels through the aperture (106) to the beam splitter (101) where thesignal is split (112 c travels opposite aperture, 112 d travels oppositethe CCD) and is then imaged onto a pair of diffraction gratings (102 a,102 b) at a grating angle in a Littrow configuration (θ_(L)). The lightis then redirected and recombined by the beamsplitter (101), and thentravels (along 112 e) through an additional lens (103) before beingimaged by the CCD (105).

FIG. 2D shows a side view (115) of system 200. Light (112 a) isgenerated by the laser (113). The light then travels along theexcitation light path (112 a) to the dichroic mirror (109) where it isredirected to the MLA (110). The light is focused by the MLA (110) ontothe area (111 a) of the sample (111) where the signal is collected backthrough the MLA (110) and through the dichroic mirror (109). Light thentravels through the rest of the transfer lens setup (114) along the path(112 b) to the SHRS (100).

A method 300 for imaging a sample using the system 200 of the presentinvention is described in detail below with reference to FIG. 3 . Asshown in FIG. 3 , method 300 is comprised of five steps. System 200 isused to collect the signal of the light product used by method 300. Oncecollected by the detector (105) inside the SHRS (100), an array offringe images is obtained (301). FIG. 4 illustrates one embodiment of afringe image (400) having rows R1-R5 and columns C1-C8. The square boxat row 3, column 5 refers to the lenslet region of interest (402) for aparticular sample. The one lenslet region of interest (shown boxed infigures) is chosen (302) and the columns are summed in the verticaldirection to provide a raw interferogram superimposed on a backgroundsignal (303). The interferogram is obtained by subtracting a polynomialfit from the cross section (304), and the spectrum is obtained by takingthe 1D Fourier transform of the interferogram (305). This allows animage and spectrum to be taken at the focal point of every microlens ina single exposure. In some embodiments, the signal may be processed by avariety of different data filters or signal processing software. In oneembodiment, a Golay filter can be used when selecting one lensletspectra.

In an alternative embodiment, the spatial heterodyne spectrometer can beof a monolithic construction, as shown in FIG. 13 . Monolithic opticsare solid state and are made from bonded components of optical materials(e.g., quartz, fused silica, BK7 or combinations thereof) where theoptical components are cemented, epoxied, or optically contacted afteralignment. The devices are more robust and stable than equivalentfree-space optical devices (e.g., where components are individuallymounted). Since the parts are directly bonded together after alignment,the entire system can be very small in size, and since it contains nomoving parts, it is much more robust than other spectrometers. It isalso less sensitive to vibration, making it ideal for many applicationswhere vibration is a concern. The monolithic spatial heterodyne Ramanspectrometer also has very high sensitivity because of a 100-fold higherlight throughput for extended sources.

In yet another embodiment, shown in FIG. 14 , the spatial heterodynespectrometer can contain, in addition to diffraction gratings 102 a and102 b, one or more additional diffraction gratings (102 c-102 f)adjacently positioned at the same Littrow wavelength such that theperiodic structured surface of each adjacently positioned grating isstepped (i.e., offset a predetermined distance) to adjust for pathlengthdifferences across a portion of or across the entire grating aperturealong the dimension parallel or perpendicular to the grating dispersion.The light travels through the aperture (106) of the SHRS (100) to thebeamsplitter (101) and then is imaged onto the adjacently positioneddiffraction gratings (102 a-102 f) at a grating angle in a Littrowconfiguration (θ_(L)). The light is then redirected and recombined bythe beamsplitter (101), and then travels through an additional lens(103) before being imaged by the CCD (105). The grating angles forLittrow configuration (θ_(L)), which are measured relative to thesurface normal of the diffraction gratings, have a useful range of about0° to about 45°, such as from about 0.01° to about 30°, such as fromabout 0.1° to about 15°, such as from about 0° to about 10°, such asfrom about 0° to about 5°. This is useful in fixed systems where thegrating angle cannot be changed, such as a monolithic SHS. Furthermore,each of the adjacently positioned diffraction gratings can be rotated aspreviously described in a previously discussed embodiment to expand thespectral range. Each of the diffraction gratings could have the same ordifferent groove densities. Each of the diffraction gratings could havea different Littrow wavelength.

In still another embodiment, rather than utilizing stepped diffractiongratings as in FIG. 14 , FIG. 17 shows that a refractive optic (i.e.,refractive corrective optic—RCO), can be added between the beam splitterand grating (G1 and G2) faces to adjust the pathlength based onrefractive index. Note that the grating surface where light dispersionoccurs is the surface facing the beam splitter in the illustration. Theapproach of using an RCO is another concept for adjusting for pathlengthdifferences across a portion or across the entire grating along thedimension parallel or perpendicular to the grating dispersion. Thisillustration shows the RCO for adjusting pathlength differences alongthe dimension parallel to the grating dispersion, which would be thepreferred implementation for a non-tilted grating SHS design. It shouldbe noted that there are many ways to implement this approach. The designof the RCO would depend, in part, on the initial alignment scheme of theSHS. It should also be noted that the grating aperture refers to thegrating area that is useable in the SHS. For example, the monolith SHSdevices shown in FIG. 13 have grating apertures of approximately 17×17mm².

The present invention may be better understood by reference to thefollowing examples.

Example 1

A continuous 532 nm laser (MLL-FN-532-300 mW, OptoEngine, LLC)illuminated a 25 mm diameter, 550 nm longpass dichroic mirror (DMLP550,ThorLabs, Inc.), which directed the on-axis illumination beam throughthe MLA (MLA) (19-0055, SUSS MicroOptics), where each lenslet focusedthe laser to a spot on the sample. The surface of the sample was located1.5 mm away at the focal point of the f/16 MLA. In this way, the samplewas illuminated in epifluorescent geometry with an array of spots. TheAiry disk diameter is 20 microns.

The fused silica MLA had a 4×4 mm overall size with 100-micron diametercircular lenslets packed in a square grid. The MLA had chromiumapertures to block light between the lenslets and had an antireflectivecoating at 780 nm. Samples were illuminated with about 300 μW perlenslet.

Each individual MLA lenslet collected the signal from each illuminatedspot. The relay lens, a Nikon™ AF NIKKOR 80-200 mm f/4.5-5.6, was usedto image the back surface of the MLA onto the SHRS gratings. Theplacement and focal setting of the relay lens were changed depending onthe desired resolving power per lenslet. For a fixed grating linedensity, the relay lens magnification dictated the lenslet beam size onthe grating face, and therefore, the resolving power per lenslet.Magnifications of 8× and 10× were used. Using 300 lp/mm gratings for 10×magnification, the resolving power per lenslet was 594, whichcorresponds to about 32 cm⁻¹ FWHM, and for 8× magnification, theresolving power per lenslet was 450, which corresponds to about 40 cm⁻¹FWHM. A spatial filter with 4 mm diameter was located at the focal pointof the relay lens. This spatial filter isolates the lenslet beams fromother background signals as described by Tiziani et. al.

The SHRS was equipped with a 25 mm N-BK7 non polarizing 50:50 cubebeamsplitter (B5013, ThorLabs, Inc.) and a pair of 300 lp/mm gratingsblazed at 500 nm (#64-403, Edmund Optics). An iris at the input aperturelimited the size of the illuminated area on the gratings to 18 mm. TheSHRS was equipped with four 532 nm longpass filters (LP03-532RE-25,Semrock RazorEdge®), a 550 nm longpass filter (FEL0550, ThorLabs, Inc.)and a 581 nm shortpass filter (581FD525, Knight Optical Ltd.) to removestrong Rayleigh scatter from the laser and to keep incoming signalwithin the SHRS spectral range. Inside the SHS, a fused silica f/4.5lens with 105 mm focal length (UV-MICRO-APO 111032, Coastal OpticalSystems, Inc.) imaged the interferogram with 1.2× magnification onto athermoelectrically cooled back-illuminated UV-enhanced CCD detector with2048×512, 13.5 μm pixels (PIXIS-2048 2KBUV, Teledyne PrincetonInstruments). A spatial filter placed one focal length from theinterferogram imaging lens was used to block higher grating orders.Images were acquired in Lightfield® 4.10 software with 100 kHz ADC gainhigh and in the low noise setting. The CCD was cooled to −70° C.

To obtain the spectra, the raw image was first imported into MATLAB® andthe region of interest was selected. The columns of the selected regionwere summed in the vertical direction to give a raw interferogramsuperimposed on a background signal. To remove this background, apolynomial was fit and subtracted from the cross section. This correctedcross-section was then Fourier transformed to reveal the Raman spectrum.A 13 mm pellet die (#3619, Carver) was used to press a variety ofheterogeneous pellet samples. All samples were prepared with theintention of keeping the constituents spatially separate. A diamond (Kit#458200, Ward's® Science) was pressed into a Potassium Perchlorate(#11630, 99% anhydrous, Alfa Aesar®) pellet.

FIG. 8 shows diamond in perchlorate measured with the MLA-SHRS. Thesample was illuminated with 300 μW/lenslet for 10 minutes. The rawinterferogram image at the top is labeled with the one-lenslet regionsof interest. Below this are the spectra corresponding to each region ofinterest, offset for clarity. Region A shows only the diamond Raman bandat 1332 cm⁻¹; region B shows both potassium perchlorate and diamondRaman bands; and region C shows only the 941 cm′ Raman band of potassiumperchlorate. This demonstrates that spatial-spectral separation isretained in the horizontal dimension using the MLA-SHRS. The resolvingpower per lenslet is 594, which corresponds to about 32 cm⁻¹ FWHM.Inlaid with the spectra are the corrected interferograms for each regionof interest. Region A is the higher frequency diamond Raman fringes;region B shows a mixture of the diamond and potassium perchlorate Ramanfringes; and region C shows the lower frequency potassium perchlorateRaman fringes.

FIG. 12 describes the tradeoff between spectral resolution and areaviewed on the sample. Resolving power of the SHRS is proportional to thenumber of grooves illuminated on the gratings. The diamond Ramanspectrum was collected using the MLA-SHRS with 300 lp/mm gratings withthe MLA magnified 8× onto the SHS gratings. The sample was illuminatedwith 300 μW/lenslet for 3 minutes. With this system, resolutions from 42cm⁻¹ to 7 cm⁻¹ depending on the number of lenslets chosen to Fouriertransform could be obtained. The larger the width (more lenslets) of theregion of interest that underwent a Fourier transform, the smaller theFWHM in the recovered spectrum. Inlaid in FIG. 12 , the diamond Ramanpeak at 1332 cm⁻¹ plotted for 1, 3, 5 and 7 lenslet regions of interest.One lenslet corresponds to 100-micron diameter area viewed on thesample.

Example 2

Further using the operational setup shown in Example 1, a bilayer pelletof sodium sulfate and potassium perchlorate (239313, Sigma Aldrich) wassampled. FIG. 9 shows a sodium sulfate/potassium chlorate bilayer pelletmeasured with the MLA-SHRS. The sample was illuminated with 300μW/lenslet for 10 minutes. The raw interferogram image is labeled withthe one-lenslet regions of interest and the corresponding spectra areshown below. Spatial-spectral separation is demonstrated again as regionA shows only the sodium sulfate band at 990 cm⁻¹; region B shows bothpotassium perchlorate and sodium sulfate Raman bands; and region C showsonly the 941 cm⁻¹ Raman band of potassium perchlorate. The resolvingpower per lenslet was 594 (which corresponds to about 32 cm⁻¹ FWHM),which was enough to resolve the closely spaced bands. The correctedinterferograms for each region of interest are inlaid with the spectra.Region A is the higher frequency sodium sulfate Raman fringes; region Bshows a mixture of the sodium sulfate and potassium perchlorate Ramanfringes; and region C shows the lower frequency potassium perchlorateRaman fringes.

Example 3

Further, using the operational setup shown in Example 1, a bilayerpellet of sodium nitrate and potassium perchlorate (347663, SigmaAldrich) was sampled. FIG. 10 shows a sodium nitrate/potassiumperchlorate bilayer pellet measured with the MLA-SHRS. The sample wasilluminated with 300 μW/lenslet for 3 minutes. The raw interferogramimage was labeled with the one-lenslet regions of interest and thecorresponding spectra are below. Region A shows only the sodium nitrateband at 1068 cm⁻¹; region B shows both potassium perchlorate and sodiumnitrate Raman bands; and region C shows only the 941 cm⁻¹ Raman band ofpotassium perchlorate. The resolving power per lenslet was 450, whichcorresponds to about 40 cm⁻¹ FWHM.

Example 4

Further, using the operational setup shown in Example 1, a bilayerpellet of acetaminophen and ammonium nitrate (A7085 and 256064, SigmaAldrich) was sampled. FIG. 11 shows an acetaminophen/ammonium nitratebilayer pellet measured with the MLA-SHRS. The sample was illuminatedwith 300 μW/lenslet for 3 minutes. The raw interferogram image islabeled with the one-lenslet regions of interest and the correspondingspectra are below. The changing concentrations of acetaminophen andammonium nitrate are seen across each lenslet of interest. Region Ashows the acetaminophen Raman spectrum; regions B and C show the Ramanbands of both acetaminophen and ammonium nitrate at differentconcentrations; and region D shows only the ammonium nitrate peak at1043 cm⁻¹. The resolving power per lenslet was 450, which corresponds toabout 40 cm⁻¹ FWHM.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents, and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

Example 5

For Raman imaging measurements of an acetaminophen sample, FIG. 16 showsa 4×4 section of lenslets having spatially isolated interferograms witha cross-hatched pattern, which is indicative of a rotation of the Fizeaufringes and a phase shift along the y-axis. The Littrow wavenumber ofthe SHRS was set to 1000 cm⁻¹ as indicated by σ_(L) in the spectra, soRaman bands above and below Littrow rotate the fringes in oppositedirections. Raman spectra (right) for the regions A-C, labeled on thedetector fringe image, were obtained via a 2D Fourier transform method.Using the MLA-SHRS for Raman imaging, there is a tradeoff betweenspectral resolution and number of lenslets used in the horizontaldirection (e.g., grating grooves illuminated). For example, the spectraproduced by region A for a single lenslet, and region B which coversfour lenslets vertically, produce spectra with the same resolution, ˜42cm⁻¹, because both regions illuminate the same number of gratinggrooves. However, the spectrum produced by region C, viewing 4 lensletshorizontally, has four times higher resolution, —11 cm⁻¹, because 4times as many grating grooves were illuminated. The intensity scaleswith the number of lenslets viewed, regardless of the direction.

1. An optical apparatus for producing and simultaneously acquiring atleast two spatially isolated Fizeau fringe patterns each having anencoded light product formed as a result of receiving a light productfrom at least one object, wherein said optical apparatus comprises: atleast one spatial heterodyne spectrometer constructed to receive atleast two light input beams and produce, from each said light inputbeam, two corresponding light output beams of said spatially isolatedFizeau fringe patterns; wherein the at least one spatial heterodynespectrometer comprises a beam splitter for directing the light productand subsequently recombining, and one or more diffraction gratings,wherein the diffraction gratings are configured to adjust a wavelengthof the light product; an optical element for receiving the light productfrom the at least one object and produce the at least two light inputbeams to the at least one spatial heterodyne spectrometer; a means fordirecting at least one excitation source to interact with the at leastone object to produce the light product; and at least one detector arrayand at least one optical element for imaging the at least two spatiallyisolated Fizeau fringe patterns.
 2. The apparatus of claim 1, whereinthe at least one excitation source is a light emitting diode; a pulsed,continuous wave, or semi-continuous wave laser source; a coherentsource; an incoherent source; or a combination thereof.
 3. The apparatusof claim 1, wherein the optical element is a lens, an array of lenses, amicrolens array, a multi-component lens, an optical fiber, an opticalfiber bundle, a coherent fiber imaging bundle, a hollow fiber waveguide,a waveguide, a fiber conduit, a fiber faceplate, a mirror, a mirrorarray, a telescope, or a combination thereof.
 4. The apparatus of claim1, wherein the light product from the at least one object comprisesRaman scattering wavelengths, atomic emission wavelengths, laser inducedbreakdown emission wavelengths, or a combination thereof.
 5. Theapparatus of claim 1, wherein the at least one detector array is acharge coupled device, an intensified charge coupled device, a frametransfer charge coupled device, an electron multiplying charge-coupleddevice, a complementary metal oxide semiconductor (CMOS) sensor, or acombination thereof, wherein the at least one detector array isconfigured to collect wavelengths.
 6. The apparatus of claim 1, whereinthe one or more diffraction gratings is configured to a predeterminedLittrow angle to select a heterodyne wavelength.
 7. The apparatus ofclaim 1, wherein the one or more diffraction gratings is configured to apredetermined rotation angle to adjust an angle of the at least twospatially isolated Fizeau fringe patterns.
 8. The apparatus of claim 1,wherein the encoded light product comprises Raman wavelengths, emissionwavelengths, or a combination thereof.
 9. The apparatus of claim 8,wherein the at least one spatial heterodyne spectrometer is configuredas a spatial heterodyne Raman spectrometer.
 10. The apparatus of claim9, wherein the spatial heterodyne Raman spectrometer is monolithic, freespace optics, or a combination thereof.
 11. The apparatus of claim 9,further comprising one or more blocking filters, one or more band passfilters, or a combination thereof, wherein the one or more blockingfilters and the one or more band pass filters are configured to removelight outside of the Raman wavelengths.
 12. The apparatus of claim 8,wherein the at least one spatial heterodyne spectrometer is configuredas a spatial heterodyne laser-induced breakdown spectrometer, whereinthe spatial heterodyne laser-induced breakdown spectrometer ismonolithic, free space optics, or a combination thereof.
 13. (canceled)14. The apparatus of claim 12, further comprising one or more blockingfilters, one or more band pass filters, or combination thereof, whereinthe one or more blocking filters and the one or more band pass filtersare configured to remove light outside of the emission wavelengths. 15.The apparatus of claim 1, wherein the at least one spatial heterodynespectrometer is configured with one or more prisms to further increasean acceptance angle, or wherein at least one of the one or morediffraction gratings has a stepped configuration.
 16. (canceled)
 17. Theapparatus of claim 1, further comprising a refractive corrective optic.18. The apparatus of claim 1, wherein the encoded light product isdecoded using a Fourier transform method or other decoding methods. 19.(canceled)
 20. A device for imaging a sample comprising: an excitationsource; a spatial heterodyne spectrometer; and a microlens array;wherein the microlens array and a surface of the sample to be imaged arearranged in parallel, and wherein the microlens array collects lightfrom different regions of the surface of the sample.
 21. The device ofclaim 20, wherein the excitation source is a light emitting diode, alaser source, a coherent source, an incoherent source, or a combinationthereof, wherein the spatial heterodyne spectrometer is a laser-inducedbreakdown spectrometer, or wherein the spatial heterodyne spectrometeris configured to receive Raman wavelengths from the sample. 22-23.(canceled)
 24. The device of claim 21, further comprising one or moreband pass filters, the one or more band pass filters being configured toremove light outside of the Raman wavelengths.
 25. The device of claim20, further comprising one or more blocking filters, a charge-coupleddevice configured to collect Raman wavelengths, a diffraction grating ordispersive prism configured to adjust Raman wavelengths, or acombination thereof. 26-27. (canceled)
 28. The device of claim 25,wherein a grating angle of the diffraction grating or the dispersiveprism is adjustable, or wherein the spatial heterodyne spectrometercontains one or more simple wedge prisms to adjust an acceptance angleof light from the sample.
 29. (canceled)
 30. The device of claim 28,further comprising transfer optics comprised of one or more collectionlenses or apertures for collimating the light from the microlens arrayto within the acceptance angle of the spatial heterodyne Ramanspectrometer, further wherein a relay lens is positioned two focallengths from the microlens array and two focal lengths from the apertureof the spatial heterodyne Raman spectrometer, wherein a center of themicrolens array is aligned with a center of the relay lens. 31-44.(canceled)
 45. A device comprising: an excitation source; a spatialheterodyne spectrometer comprised of a beam splitter and a pair ofdiffraction gratings; and one or more additional diffraction gratings.46. The device of claim 45, wherein the excitation source is a lightemitting diode, a laser source, a coherent source, an incoherent source,or a combination thereof, or wherein the spatial heterodyne spectrometeris a monolithic spatial heterodyne spectrometer.
 47. (canceled)
 48. Thedevice of claim 45, wherein one or more additional diffraction gratingsare stacked sequentially one above another, further wherein spacers aredisposed between each of the additional diffraction gratings, furtherwherein a different Littrow wavelength for each of the additionaldiffraction gratings is selected by adjusting a grating angle of each ofthe additional diffraction gratings individually relative to each otherand relative to the grating angle of the pair of diffraction gratings inthe spatial heterodyne spectrometer.
 49. (canceled)
 50. The device ofclaim 48, wherein each of the additional diffraction gratings has aunique groove density relative to each other and relative to the groovedensity of the pair of diffraction gratings in the spatial heterodynespectrometer. 51-55. (canceled)